IT Home (IT之家) May 17 news - It turns out that interstellar comet 3I/ATLAS almost got named comet 3I/Rubin. Researchers have discovered that the large survey telescope accidentally observed this visitor from interstellar space more than a week before the comet was officially discovered.

Comet 3I/ATLAS was officially confirmed on July 1, 2025, by the Asteroid Terrestrial-impact Last Alert System (ATLAS (小行星陆地撞击最后预警系统)). This system is a network of fully automated telescopes distributed across Hawaii, Chile, and South Africa. Ten days earlier, the Vera C. Rubin Observatory (薇拉・C・鲁宾天文台), also located in Chile, had already entered its scientific verification phase in preparation for full operation later that year. The primary goal of this phase was to calibrate the 8.4-meter telescope and its supporting equipment, ensuring all instruments functioned properly.

Led by Colin Orion Chandler (科林・奥赖恩・钱德勒) from the University of Washington, the team was curious whether the Rubin Observatory had captured the comet before it was officially named, and thus began sorting through the observatory's commissioning data. Sure enough, they found that the Rubin telescope had imaged comet 3I/ATLAS on the very night of June 20, when it took its first test observation images. This is a full ten days earlier than the time when the ATLAS system discovered it.

This data mining effort was by no means an easy task. At present, the Rubin Telescope has a well-planned, standardized process specifically designed for astronomers to collect and process observational data—what the industry calls a data processing pipeline. However, during the scientific validation phase, this pipeline had not yet been activated. The Chandler team had to build their own dedicated data processing program in order to successfully retrieve the relevant observational data.

Chandler speculated that if the Rubin Observatory had launched scientific validation a few weeks earlier, its supporting data processing system could have been put into use sooner, and it would have had a very good chance of discovering this interstellar comet before July 1.

The researchers also found that between June 21 and July 2, the Rubin Telescope captured this interstellar comet nine more times, and from July 2 to July 20, it captured it multiple times. These images clearly confirmed that the comet had already been active—with a distinctly visible coma—long before it was detected by the ATLAS system. The coma is a cloud of dust and gas formed when a comet approaches the Sun, heats up, and releases volatiles from its surface, enveloping its nucleus.

According to the plan, the Rubin Observatory is expected to discover up to 10,000 new comets during its decade-long Legacy Survey of Space and Time (LSST) core observation mission. The early detection of comet 3I/ATLAS has confirmed previous estimates: the telescope is expected to observe an average of one interstellar comet entering the solar system per year. Although 3I/ATLAS ultimately did not bear the Rubin name, future interstellar comets will certainly be named after it.

Joint observations by Jupiter-bound spacecraft

According to IT Home, comet 3I/ATLAS has not yet left the solar system, and multiple orbiting probes continue to monitor it, continuously uncovering new observational data. In October 2025, the comet moved behind the Sun, making it unobservable from Earth's perspective. This coincided with its arrival at perihelion (closest approach to the Sun), also the period of highest cometary activity, during which spacecraft observations played a crucial role.

A research team from the Southwest Research Institute in the United States operates ultraviolet spectrometers on two probes: one aboard the European Space Agency's Jupiter Icy Moons Explorer (JUICE) and the other aboard NASA's Europa Clipper. Both probes are currently en route to Jupiter. The team confirmed that the two probes jointly completed observations of comet 3I/ATLAS in late 2025.

Kurt Retherford from the Southwest Research Institute said, “When this comet passed between the two probes, we achieved an informal coordination of observations between the two spacecraft.”

The Jupiter Icy Moons Explorer observed the sunlit side of the comet, while the Europa Clipper captured the comet's dark side, allowing researchers to simultaneously observe the same gaseous substances released by the comet from two different perspectives.

The ultraviolet spectrometers aboard the two probes jointly detected elements such as hydrogen, oxygen, and carbon. These substances originated from gas molecules released by the comet's nucleus, which were broken down into individual atoms under solar ultraviolet radiation. The observational data showed that this interstellar comet has a much higher carbon content than native comets in the solar system, a finding consistent with previous observations by the James Webb Space Telescope that indicated high carbon dioxide levels in this comet.

Philippa Molyneux from the Southwest Research Institute stated, “By analyzing the ratio of water ice to dry ice (solid carbon dioxide), we can compare the material composition of this interstellar comet with that of solar system comets, and thereby determine whether its home planetary system is similar to ours.”

Previously, multiple space exploration missions and ground observation stations have collected a vast amount of observational data on the 3I/ATLAS comet. The latest joint observation results have further enriched the relevant research materials. The scientific community has now determined that the comet's nucleus is about 1 kilometer in diameter, traveling at a speed of up to 140,000 miles per hour (61 kilometers per second). This extremely high velocity indicates that the comet is at least 7 billion years old, possibly up to 12 billion years, having accelerated over time through multiple close encounters with other stars during its long history.

The observational analysis paper on this comet by the Rubin Observatory was officially published in The Astrophysical Journal Letters on April 20.

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